In recent years, improvement in vehicle fuel efficiency has been an urgent issue in the automobile industry, in view of reducing CO2 emission causative of global warming. In addition to drastic efforts for reducing the CO2 emission by using substitutive fuels, there are growing needs for measures such as improving mechanical efficiencies of engine, transmission and so forth, and reducing weight of vehicle body. On the other hand, in the situation directed to more tight crash safety regulations, another important issue is to develop a vehicle body excellent in vehicle safety performance.
It is however necessary to use a lot of reinforcing components or to thicken vehicle components, in order to improve the vehicle safety performance only by using low-strength steel sheet which configures vehicle bodies, so that it is not easy to harmonize the improvement with the light weight body.
For the purpose of harmonizing the light weight body and the improvement in vehicle safety performance, efforts have been made on use of high-strength steel sheet for vehicle components such as frame. For example, much of conventional vehicle components have been made of a steel sheet with a tensile strength of 440 MPa class, whereas recent vehicle components have increasingly adopted a steel sheet of 590 MPa class, and have become to adopt even a steel sheet of 980 MPa class or above.
The high-strength steel sheet has, however, encountered increased opportunities of shape fixation failure (spring-back) and wrinkle in the process of press forming (bending) as the strength of the steel sheet increases, gradually making it difficult to ensure dimensional accuracy of the vehicle components. In addition, decrease in ductility, accompanied by improved strength of the steel sheet, will increase a risk of breakage in the process of press forming.
It is therefore not always easy for the vehicle components composed of the high-strength steel sheet to harmonize performances and productivity of vehicle body, as compared with the conventional vehicle components making much use of the low-strength steel sheet, and this is understood as one of hindrances against use of the high-strength steel sheet for the vehicle components, under requirements of shortened period of development and reduction in manufacturing cost.
On the other hand, as methods of enhancing the crash safety performance of the vehicle components without using the high-strength steel sheet, there have been proposed methods of strengthening the entire portion of, or a part of the components, typically by hot press forming or induction hardening (see Patent Literatures 1, 2, for example). The methods are, however, applicable to a limited range of components, since some vehicle components are not suitable for the hardening due to their geometries, and also since some new equipment need be introduced.
Still another proposal relates to use of laser as a heat source of annealing (see Patent Literature 3, for example). The laser is, however, available only in a narrow range of heating, and therefore needs a long duration of annealing, which is not practical due to difficulty in obtaining a satisfactory effect.